The present invention relates generally to a bipolar type semiconductor integrated circuit, and more particularly, to a basic logic circuit adoptable in a large scale integrated circuit (LSI) chip.
Example types of output circuit of such conventional circuits are shown in FIGS. 1(A), 1(B) an 1(C). These Figures show such examples, respectively, which all consist of output transistors, resistors and diodes. In these Figures, the symbols Vcc and A,B, . . . denote power source and output terminal, respectively. On the respective output terminals will develop a low-level output voltage (will be referred to as L level or simply as L hereinbelow) or a high level output voltage (will be referred to as H level or simply as H hereinbelow) by turning on or off the output transistor corresponding to the logic operation results. However, in many cases, the current through the output terminal is bi-directional as shown by the arrows. Namely, in case the output voltage is at H level, the current will flow out of the output transistor, while for a L level output voltage, the current will flow into the same output transistor. Because of such bi-directionality, the logic circuit including this type of output circuit cannot function practically to perform the wired logic operation as the outputs are interconnected between them. That is to say, when the outputs are shorted between them, the current will flow from the H output to the L one if one of the outputs is at H level, while the other is at L level. This is not only logically invalid but also will generally cause the failure as the voltage value on the L output is higher than the permissible value.
Since in the recent LSI circuit, it is essential to improve the circuit function and reduce the manufacturing cost by integrating in a small semiconductor chip as many logic composed by basic logic circuits as possible, therefore, it is necessary to simplify the basic logic circuit to the possible degree as well as to minimizze the total number of the basic logic circuits used in a chip. Further interconnection between the outputs as mentioned above will provide means for performing a logic operation, namely, an output logic. For instance, a wired-OR logic may be provided by simply short-circuiting the output terminals between them, which will result in omission of one basic OR gate circuit. This method is optimum in the respect of minimizing the total number of the basic logic circuits as mentioned above. But, because of the reason as set forth in the preceding, such method could not be carried out for the types of output shown in FIGS. 1(A), 1(B) and 1(C).